There is a great need for large area solid state light sources for a series of applications, especially in the file of display elements and lighting engineering. The demands cannot be fully satisfactorily met by any of the existing technologies. Electroluminescent devices such as light-emitting diodes represent an alternative to conventional display and lighting elements. Electroluminescent devices are optoelectronic devices where light emission is produced in response to an electrical current through the device. The physical model for EL is the radiative recombination of electrons and holes. Both organic and inorganic materials have been used for the fabrication of LEDs. Inorganic materials such as ZnS/Sn, Ga/Bs, Ga/As have been used, e.g. in semiconductor lasers, small area displays, and LED lamps. However, the drawbacks of inorganic materials include difficulties to process and to obtain large surface areas and efficient blue light.
Organic materials, which includes both small molecules and polymeric materials, offer several advantages over inorganic materials for LEDs, such as simpler manufacturing, low operating voltages, the possibility of producing large area and full-color displays. Conjugated polymers such as poly(phenylvinylene) (PPV) were first introduced as EL materials by Burroughes, et al. in 1990 (Burroughes, J. H. Nature 1990, 347, 539-41). Tremendous progress has been made since then to improve the stability, efficiency, and durability of polymeric LEDs (Bernius, M. T., et al., Adv. Mater. 2000, 12, 1737). Organic LED (OLED) represents an alternative to the well established display technologies based on cathode-ray tubes and liquid crystal displays (LCDs), especially for large area displays. OLED has been demonstrated to be brighter, thinner, lighter, and faster than LCDs. Moreover it requires less power to operate, offers higher contrast and wide viewing angle (>165 degree), and has great potential to be cheaper to make, especially the polymer-based LEDs (PLED).
The OLED technology has stimulated intensive research activities across all disciplines. Currently, great efforts in materials research have been focused on novel materials for full-color flexible displays. Full-color displays require three basic colors, red, green and blue, and flexible substrates require low temperature and easy processing of the organic materials. PLED devices show great promise in meeting both requirements, since the emission color can be tailored by modulation of the chemical structures and the solution processing permits for micro-patterning of the fine multicolor pixels via inkjet printing technique (Yang, Y., et al., J. Mater. Sci.: Mater. Elecron., 2000, 11, 89). However, processable, stable, and efficient blue light-emitting organic materials are still highly desirable to meet the challenge. Blue light requires wide energy band. With blue light-emitting polymers as primary materials, it is possible to produce other colors by a downhill energy transfer process. For instance, a green or red EL emission can be obtained by doping a blue EL host material with a small amount of green or red luminescent material.